Field of the Invention
The present invention relates to a capacitance type transducer that performs transmission and reception of an acoustic wave such as an ultrasonic wave (in this specification, transmission and reception means at least one of transmission and reception), a manufacturing method for the capacitance type transducer, and a subject information acquiring apparatus such as an ultrasonic image forming apparatus including the capacitance type transducer. In this specification, the acoustic wave includes waves called sound wave, ultrasonic wave, and photoacoustic wave. However, the acoustic wave is sometimes represented by the ultrasonic wave. The photoacoustic wave is an acoustic wave generated inside a subject by irradiation of light (an electromagnetic wave) such as a visible ray or an infrared ray to the inside of the subject.
Description of the Related Art
A CMUT (Capacitive Micromachined Ultrasonic Transducer), which is a capacitance type ultrasonic transducer, has been proposed for the purpose of performing transmission and reception of an ultrasonic wave. The CMUT is manufactured using a MEMS (Micro Electro Mechanical Systems) process to which a semiconductor process is applied.
A schematic diagram of a cross section of an example of a CMUT (a transmitting and receiving element) is illustrated in FIG. 19 (see A. S. Ergun, Y. Huang, X. Zhuang, O. Oralkan, G. G. Yarahoglu, and B. T. Khuri-Yakub, “Capacitive micromachined ultrasonic transducers: fabrication technology,” Ultrasonics, Ferroelectrics and Frequency Control, IEEE Transactions on, vol. 52, no. 12, pp. 2242-2258, December 2005). A structure including a first electrode 102 and a second electrode 103 opposed to a vibrating film 101 across a gap (a cavity) 105 is set as one set and referred to as cell. The vibrating film 101 is supported by a supporting section 104 formed on a chip 201. A direct-current voltage generating unit 311 is connected to the first electrode 102. A predetermined direct-current voltage Va is applied to the first electrode 102. The second electrode 103 is connected to a transmission and reception circuit 312 and set to fixed potential near the GND potential. Consequently, a potential difference of Vbias=Va−0 V is generated between the first electrode 102 and the second electrode 103. When a value of Va is adjusted, a value of Vbias coincides with a desired potential difference (approximately several tens volts to several hundred volts) determined by a mechanical characteristic of cells of the CMUT. When an alternating-current driving voltage is applied to the second electrode 103 by the transmission and reception circuit 312, alternating-current electrostatic attraction is generated between the first and second electrodes 102 and 103. An ultrasonic wave can be transmitted by vibrating the vibrating film 101 at a certain frequency. The vibrating film 101 receives the ultrasonic wave and vibrates, whereby micro current is generated by electrostatic induction in the second electrode 103. It is possible to extract a reception signal by measuring a current value of the micro current using the transmission and reception circuit 312. Note that, in the above description, a direct-current voltage generating unit 311 is connected to the first electrode 102 and the second electrode 103 is connected to the transmission and reception circuit 312. However, the transmission and reception circuit 312 may be connected to the first electrode 102 and the second electrode 103 may be connected to the direct-current voltage generating unit 311.
In general, an electrode included in a CMUT includes a metal thin film. A layer containing silicone, through which an ultrasonic wave is easily transmitted, as a main component is formed on the CMUT. The silicone has a high insulation property. Electric safety can be secured by insulation resistance. However, since the permeability of water vapor is high, the water vapor sometimes intrudes into a wire in the CMUT. Consequently, corrosion of the wire occurs because of the water vapor and ionized or micronized substances permeating together with the water vapor. A problem of reliability such as deterioration in the sensitivity of the CMUT sometimes occurs. Therefore, it is necessary to reduce the intrusion of the water vapor from the outside while minimizing the influence on a transmission and reception characteristic of the CMUT. Depending on a use of the CMUT, a packaging size needs to be kept within a small region. Therefore, there is a demand to reduce the intrusion of the water vapor, which causes the corrosion of the wire in the CMUT, and set the packaging size as close as possible to the size of a substrate to reduce the size of the CMUT.
Therefore, it is an object of the present invention to provide a capacitance type transducer that can reduce occurrence of corrosion of a wire due to intrusion of substances from the outside and has reduced influence on a transmission and reception characteristic.